The nature of dark matter is one of the most important outstanding problems in 
physics and cosmology.  From observing astrophysical objects, we have learned 
that dark matter represents some 25% of the total energy of the universe, while 
ordinary matter weighs in at only about 4%.  Although they make up a large 
fraction of the energy of the universe, dark matter particles interact with 
ordinary matter only very weakly, making their detection via interactions with 
ordinary matter difficult.  We review theories of dark matter and discuss the 
ways physicists and cosmologists are looking for dark matter through its 
signatures in the sky in gamma rays and cosmic rays, underground in direct 
detection experiments, and at the Large Hadron Collider.